Because of aggressive downsizing of transistors for enhanced performance, reduced power and increased packing density, it is sometimes desirable to have the core logic of a integrated circuit operate at a different power supply voltage than some of the peripheral circuitry which is used to interface with external circuits. The aggressive scaling of gate oxide thicknesses to maintain performance can result in potentially large electric fields on the gate dielectric of the peripheral circuitry due to the fact that the peripheral circuitry interfaces with external circuitry that operates at higher voltages than the core logic. This can result in a serious reliability problem for the overall circuit. To avoid this, a gate oxide that is thicker in the peripheral circuitry than in the logic core is needed.
One prior art method for obtaining dual gate oxide thicknesses is called "split-gate". In one "split gate" process, an initial oxide is grown followed-by photolithographically masking areas where thick oxides are desired, then etching the grown oxide in areas where the thin oxide is required. The photoresist is then removed via a clean-up process that may include ashing and a final oxidation is performed to grow the thin oxide and anneal and slightly thicken the oxide already grown in the thick oxide areas. One of the primary drawbacks of this approach is a higher defect density (resulting in low yield) for the thicker gate oxide due to extra processing for the thicker oxide and exposure to resist and resist removal steps.
Another prior method for forming dual gate oxide thicknesses uses nitrogen implants into the silicon to retard oxidation rates in selected areas. Using this technique, areas with the nitrogen implant are grown to one thickness while those without the implant are grown thicker in a single oxidation step. While this method allows one to selectively control oxide thicknesses, there are several disadvantages. Nitrided oxides exhibit lower carrier mobility, higher oxide charge, and higher interface trap densities than pure thermal oxides. Since it is desirable to retard oxidation in the logic core, nitrogen would be implanted into this area where high performance is required, possibly resulting in degraded performance. Additionally, at higher doses, surface roughness can be severely degraded. This can pose reliability problems in addition to degrading performance.